A contemplated application of the present invention is in ion sources in an ion implanter that may be used in the manufacture of semiconductor devices or other materials, although many other applications are possible. In such an application, semiconductor wafers are modified by implanting atoms of-desired dopant species into the body of the wafer to form regions of varying conductivity. Examples of common dopants are boron, phosphorus, arsenic and antimony. These dopants are generated in an ion source.
Typically, an ion implanter contains an ion source held under vacuum within a vacuum chamber. The ion source produces ions using a plasma generated within an arc chamber. The plasma in the arc chamber is struck using potential differences and a source of thermal electrons. The thermal electrons may be generated using one of a number of different arrangements such as a Freeman source or a Bernas source (including indirectly heated cathodes).
In a typical Bernas source, thermal electrons are emitted from a cathode, accelerated under the influence of an electric field and are constrained by a magnetic field to travel along spiral paths towards a counter-cathode. Interactions with precursor gas molecules within the arc chamber produces the desired plasma.
Plasma ions are extracted from the arc chamber via an aperture provided in a front plate. In an “ion shower” mode, the ions travel to implant in a target such as a semiconductor wafer. Alternatively, the extracted ions may be passed through a mass analysis stage such that ions of a desired mass and energy are selected to travel onward to implant in a semiconductor wafer. A more detailed description of an ion implanter can be found in U.S. Pat. No. 4,754,200.
The ion source will comprise the arc chamber to contain the plasma. Chamber walls and a front plate like that shown in FIGS. 1 and 2 enclose the arc chamber. This two-piece construction assembles to form a slot-like aperture to allow ions to be extracted from the arc chamber. Tongue and groove arrangements, shown at A, are provided to facilitate alignment of the two parts of the front plate. An extraction electrode assembly is generally provided in front of the aperture to extract ions from the ion source, and the front plate may form one of the electrodes of that assembly.